Shape adjusting mechanisms for back rests are well known. Such mechanisms typically include a shaping element, also referred to as a panel or "lumbar basket", which is mounted for displacement along a guide track. The lumbar basket may have various configurations. A basic construction involves a pair of brackets displaceable along an axis of the guide track, resilient axial ribs joining the brackets and resilient transverse ribs fixed centrally to the axial ribs with free ends extending laterally to either side of the axial ribs to provide a cushioning effect. Various mechanisms can be used to draw the brackets together in order to flex the lumbar basket from a relatively flat rest state to various bowed states. Various mechanisms can also be used to displace the lumbar basket axially along the track. Thus, the curvature of the lumbar basket and its position within a back rest can be adjusted to provide greater comfort.
The basic lumber basket described above has a flexed profile which is essentially a segment of a circle, and consequently does not conform adequately to the curvature of a user's spine. One prior art approach to altering the basic flexed profile involves fixing a partial central rib to an upper bracket and an upper set of the transverse ribs, making the upper end of the basket more rigid. This induces greater flexing of the basket proximate to the lower bracket, providing greater comfort for many users. There are, however, shortcomings to such an approach. Making the partial rib and then fastening it to multiple components of the basic lumbar basket contributes to cost. There is also little freedom to specify the profile ultimately presented by the lumbar basket.
Another approach to altering the basic flexed profile involves stamping each axial rib with reinforcing flanges that extend partially along the length of the rib and produce lengthwise rib sections of different bending resistance that determine the profile of the lumbar basket in its flexed state. This approach provides advantages including reduced manufacturing costs over the above described approach. Despite these advantages there is a continuing need for further reductions in manufacturing costs for the lumbar basket as well as a continuing need to improve the operation and weight characteristics of the lumbar basket.
Another problem with conventional lumbar basket designs is that they may flex in the opposite direction than desired when moved from a rest state to a flexed state. This may pose a safety hazard or at the very least an annoyance to consumers if the basket suddenly snaps from the opposite flexed state. It is desirable that a lumber basket be developed that will not flex in an opposite direction.